Endodontic instrument

ABSTRACT

A method of fabricating an endodontic instrument by a machining operation is disclosed, and wherein a wire-like rod composed of a titanium alloy is advanced past a rotating grinding wheel at a relatively slow feed rate, with a sufficient depth of cut to remove all of the material on a given surface without over grinding a previously ground surface, and with the grinding wheel rotating at a relatively slow surface speed. The disclosed method is able to efficiently produce endodontic instruments having a high degree of flexibility, high resistance to torsional breakage, and with sharp cutting edges along the working length.

This application is a continuation of application Ser. No. 08/076,367,filed Jun. 14, 1993, now U.S. Pat. No. 5,527,205, which in turn is acontinuation of application Ser. No. 07/787,945 filed Nov. 5, 1991 andnow abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to a method of fabricating an endodonticinstrument adapted for use in performing root canal therapy on teeth,and which is characterized by high flexibility and high resistance totorsional breakage.

Root canal therapy is a well-known procedure wherein the crown of adiseased tooth is opened so as to permit the canal to be cleaned andthen filled. More particularly, a series of very delicate, flexible,finger-held instruments or files are used to clean out and shape theroot canal, and each file is manually rotated and reciprocated in thecanal by the dentist. Files of increasingly larger diameter are used insequence, to achieve the desired cleaning and shaping. When the canal isthus prepared, it is solidly filled with a filling material, whichtypically comprises a waxy, rubbery compound known as gutta percha. Inone procedure, the gutta percha is positioned on an instrument called acompactor, and the coated compactor is inserted into the prepared canaland rotated and reciprocated to compact the gutta percha therein. Thedentist thereafter fills the tooth above the gutta percha with aprotective cement, and lastly, a crown is fitted to the tooth.

Endodontic instruments of the described type are conventionallyfabricated by permanently twisting a stainless steel rod of triangularor square cross section. The apices of the triangular or square crosssection thus form cutting edges which spiral along the length of theinstrument. More recently, such instruments have been produced by amachining process, and wherein a cylindrical rod of stainless steel ismoved past a rotating grinding wheel, and while the rod is slowlyrotated about its axis so as to impart a desired helical configurationto the ground surface and form a spiral flute on the surface. The rod isthereafter indexed and again moved past the wheel, and these steps arerepeated as many times as are necessary to form the rod into atriangular or square cross section. By appropriate control of theprocess, helical lands may be formed between the spiral flutes asillustrated in U.S. Pat. No. 4,871,312 to Heath.

It is well-known by clinicians that inadvertent errors can occasionallyarise during root canal therapy as described above. These errors caninclude the formation of a ledge in the wall of the canal, theperforation of the canal, and a separation or fracture of theinstrument. Many of these errors which occur during the therapy of acanal have a common genesis, i.e. the basic stiffness of the stainlesssteel instruments, particularly with the respect to the instruments oflarger size. Efforts have been made to improve the flexibility ofstainless steel instruments based upon different cross sectional shapes,but without significant success.

Recently, a series of comparative tests of endodontic instruments madeof nickel-titanium (Nitinol) alloy and stainless steel were conducted.The results of the tests were published in an article entitled "AnInitial Investigation of the Bending and the Torsional Properties ofNitinol Root Canal Files", Journal of Endodontics, Volume 14, No. 7,July 1988, at pages 346-351.

The Nitinol instruments involved in the above tests were machined inaccordance with the procedure and operating parameters conventionallyused in the machining of stainless steel endodontic instruments. Moreparticularly, this standard procedure involves the following parameters:

1. Feed Rate

The rod from which the instrument is to be formed is moved axially pasta rotating grinding wheel at a feed rate of about ten inches per minute.The rod is slowly rotated about its axis as it is axially advanced so asto impart a helical configuration to the ground surface.

2. Depth of Cut

The depth of each cut is sufficient to remove all of the material on agiven surface without over grinding a previously ground surface. Forexample, in the case of an instrument triangular cross-section, the rodis moved past the wheel three times, once for each surface, with about25 percent of the diameter being removed on each cut.

3. Speed of Wheel

An aluminum oxide grinding wheel is provided which is rotated at asurface speed of about 6000 feet per minute, and the wheel has a gritsize of about 220.

The above tests demonstrated that the Nitinol instruments produced bythe described machining process exhibited superior flexibility andtorsional properties as compared to stainless steel instruments, but thecutting edges of the instruments exhibited heavily deformed metaldeposits, which rendered the instruments generally unsatisfactory foruse.

It is accordingly an object of the present invention to provide a methodof fabricating an endodontic instrument which is characterized by highflexibility and high resistance to torsional breakage.

It is another object of the present invention to provide a method ofefficiently fabricating an endodontic instrument which is composed of atitanium alloy, such as a nickel-titanium alloy, and which exhibits highflexibility and high resistance to torsional breakage, and which is alsocharacterized by sharp cutting edges.

SUMMARY OF THE INVENTION

The above and other objects and advantages of the present invention areachieved in the embodiments illustrated herein by the discovery thatwhen an endodontic instrument of titanium alloy is machined undercertain specific operating parameters, a totally satisfactoryinstrument, having high flexibility, high resistance to torsionbreakage, and sharp cutting edges, may be produced. The specificoperating parameters are not suggested by the known procedure formachining stainless steel instruments as summarized above, and indeed,the parameters which are effective in producing a satisfactoryinstrument are directly contrary to accepted practices for machiningtitanium alloys as presented in authoritative literature, note forexample the brochure entitled "RMI Titanium", published by RMI Companyof Niles, Ohio.

More particularly, the present invention involves the steps of (a)providing a cylindrical rod of metallic material which is composed of atleast about 40% titanium and which has a diameter less than about 0.06inches, and (b) axially moving the rod past a rotating grinding wheel ata feed rate of not more than about 5 inches per minute, while rotatingthe rod about its axis, and so that the wheel removes at least about 25%of the diameter of the rod at the point of maximum removal and forms ahelical surface on the rod. The grinding wheel is rotated at arelatively slow surface speed of not more than about 3000 feet perminute, and preferably not more than about 2200 feet per minute. Also,the grinding wheel has a relatively fine grit size which is greater thanabout 200 grit, and preferably greater than about 220 grit. In thepreferred embodiment, the rod is composed of an alloy comprising atleast about 40% titanium and about 50% nickel.

It is often preferred to form the rod into a triangular or square crosssectional configuration, and in such embodiments, the rod is rotatablyindexed about a rotational axis of not more than 180 degrees, andspecifically either 120 degrees or 90 degrees, and step (b) is repeatedso as to form a second helical surface on the rod. The indexing andgrinding steps are again repeated as many times as are necessary to formthe desired number of sides on the instrument.

BRIEF DESCRIPTION OF THE DRAWINGS

Some of the objects and advantages of the present invention having beenstated, others will appear as the description proceeds, when taken inconjunction with the accompanying drawings, in which

FIG. 1 is a cross sectional view of a tooth have two roots, with anendodontic instrument manufactured in accordance with the presentinvention being positioned in one of the roots;

FIG. 2 is an enlarged perspective view of the lower portion of theinstrument shown in FIG. 1;

FIG. 3 is a transverse sectional view taken substantially along the line3--3 of FIG. 2;

FIG. 4 is a view similar to FIG. 2, but illustrating a second embodimentof the instrument;

FIG. 5 is a transverse sectional view taken substantially along the line5--5 of FIG. 4;

FIG. 6 is a schematic side elevation view of a machining apparatus whichis adapted to fabricate endodontic instruments in accordance with thepresent invention; and

FIG. 7 is a top plan view of the apparatus shown in FIG. 6, andillustrating certain of the steps of the fabrication process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring more particularly to FIGS. 1-3, an endodontic instrument 10 isillustrated which comprises a shank 12 which is composed of a titaniumalloy as further described below. The shank 12 typically has a length ofabout 30 mm (1.2 inches), and it includes an outer or proximate endwhich mounts a conventional handle 14. The portion of the shankimmediately below the handle is cylindrical and has a diameter ofbetween about 0.5 and 1.6 mm (0.02 and 0.07 inches), and this shankportion includes calibrated depth markings 15 of conventional design.The shank further includes an opposite distal or pilot end 16, and aworking length 18 is defined adjacent the pilot end 16. The workinglength may be cylindrical as illustrated, or it may be slightly taperedtoward the pilot end 16 at an included angle of about one degree. Theworking length 18 may have a length of about 2 mm (0.08 inches) up tothe full length of the shank 12, i.e. about 30 mm (1.2 inches). However,in the illustrated embodiment, the working length 18 has a lengthsufficient to extend substantially the full depth of a tooth root canalas illustrated in FIG. 1, which is about 16 mm (0.63 inches). Also, thecross sectional configuration of the working length 18 is triangular andis composed of three linear surfaces 19, as best seen in FIG. 3, and sothat the apices of the triangle form cutting edges.

FIGS. 4-5 illustrate a second embodiment of an endodontic instrument 10'which may be fabricated in accordance with the present invention. Inthis embodiment, the outer peripheral surface of the working length 18'is tapered at an included angle of about one degree, and the workinglength 18' includes two continuous helical flutes 21, 22 formed in theperipheral surface. The flutes have an arcuate curvature as best seen inFIG. 5, and they have a pitch so as to define helical lands 24 on theouter periphery of the instrument. An instrument of this generalconstruction is further described in U.S. Pat. No. 4,871,312 to Heath,and pending application Ser. No. 07/679,628, filed Apr. 3, 1991.

FIGS. 6 and 7 schematically illustrate a machining apparatus forpracticing the method of the present invention. As will be furtherdescribed below, the method involves a unique machining process whichhas been found to efficiently produce endodontic instruments of the typedescribed, from a rod 30 composed of titanium alloy. Such alloystypically have a titanium content of at least about 40 percent.Nickel-titanium alloys are preferred, which typically consist of about40 percent titanium and about 50 percent nickel. In one preferredspecific embodiment, the alloy consists of 44 percent titanium and 56percent nickel and no appreciable amount of other ingredients whichcould adversely effect the purity required for endodontic instruments.

The rod 30 from which the instrument is to be fabricated isconventionally supplied from the producer in a selected diameter, whichclosely conforms to the diameter of the instrument being produced. Inthis regard, endodontic instruments are sized in accordance withestablished standards, which range from a diameter at the pilot end 16of 1.4 mm (0.062 inches-size 140) to a diameter at the pilot end 16 of0.06 mm (0.0024 inches-size 06).

In accordance with the illustrated embodiment of the present invention,the continuous rod 30 is positioned to extend through an axial feedblock 32 and an indexing block 34 of conventional well-knownconstruction. A work holding fixture 36 is positioned to support theforward end of the rod 30 adjacent the periphery of a rotating grindingwheel 38. The two blocks 32, 34 are then advanced so that the rod 30 isaxially moved past the rotating grinding wheel 36 at a slow feed rate ofbetween about 3 to 8 inches per minute, and preferably not more thanabout 5 inches per minute. Concurrently with this axial movement, theindexing block 34 serves to slowly rotate the rod 30 about its axis at acontrolled speed, which causes the ground surface 19 to assume a helicalconfiguration as described above with respect to FIGS. 2 and 3.

The rod preferably moves past the wheel only once for each groundsurface 19, and thus the rod is positioned with respect to the wheel 38such that the full depth of the cut is removed in a single pass. As bestseen in FIG. 3, the wheel preferably removes at least about 25 percentof the diameter of the rod at the point of maximum removal, which isalong a diameter which extends perpendicular to the surface 19 beingformed.

As a further aspect of the present invention, the grinding wheel 38 isrotated at a relatively slow surface speed of not more than about 3000feet per minute, and preferably not more than about 2200 feet perminute. Further, the wheel 38 is composed of a relatively fine grit,which is greater than about 200 and preferably about 220 grit. A wheelof the above grit size and which is fabricated from silicon carbide hasbeen found to be very satisfactory.

To produce an instrument of the construction illustrated in FIGS. 1-3,the grinding wheel 38 is oriented to rotate about an axis generallyparallel to the axis of the advancing rod 30, and the wheel 38 thusforms a generally flat surface 19. Also, by reason of the slow rotationof the rod about its axis, this flat surface assumes a helicalconfiguration. Where the instrument is to have a tapered working length,the axis of the index block 34 is slightly inclined with respect to therotational axis of the wheel 38, so as to provide a controlled andvariable depth of cut along the working length.

When the rod 30 has advanced past the rotating wheel 38 a distancesufficient to form the first surface 19 along the desired working lengthon the instrument, the table 39 supporting the feed block 32, the indexblock 34, and the fixture 36 is moved laterally, then axiallyrearwardly, and then laterally back to its original position asillustrated schematically in FIG. 7. Concurrently, the rod 30 isrotatably indexed about its axis. The angular extent of this rodindexing will depend upon the number of surfaces 19 desired on thefinished instrument, and where three surfaces are to be formed as seenin FIG. 3, the rod is indexed 120 degrees. The rod is then again axiallyadvanced while being slowly rotated, and so as to form the secondsurface 19. The table 39 is then again moved laterally and rearwardly inthe manner described above, and the rod 30 is rotatably indexed another120 degrees. The grinding process is then repeated to form the thirdsurface 19 of the instrument. The rod 30 may then be severed byconventional techniques, such as by axially advancing the rod and thenmoving the grinding wheel laterally through the rod. The severed rod isthen further processed in a conventional manner to form the completedinstrument as illustrated for example in FIG. 1.

As a modification of the illustrated process, the rod 30 may beinitially severed into appropriate lengths, and each length may beseparately mounted in a collet at the forward end of the indexing block34, and then machined in the manner described above.

The process as described above has been found to produce instruments ofconsistently high quality, and at commercially acceptable productionrates. Of particular significance, the process results in the formationof cutting edges at the apices of the triangular cross section, whichare sharp, and substantially free of burrs and rolled edges whichcharacterized the early instruments of titanium alloys as describedabove.

While an instrument of triangular cross section is illustrated in FIGS.1-3, it will be understood that other configurations are possible. Forexample, the instrument could have four sides which form a square incross section. In the embodiment of FIGS. 4-5, the working length 18' ofthe instrument is tapered and is composed of two helical flutes 21, 22of arcuate configuration. To fabricate this embodiment, substantiallythe same procedure as described above is followed. However, the taper ofthe working length 18' is preferably initially formed on a separategrinding machine, and the tapered blank is then mounted on a machine asshown in FIG. 6, and the axis of the wheel 38 is oriented so that thewheel lies in a plane which follows the desired helical configuration ofthe flutes 21, 22. Also, the outer periphery of the wheel is curved incross section as opposed to being flat, and so as to form the desiredarcuate configuration of the flutes 21, 22. Since the instrument asillustrated has two flutes, the rod is indexed 180° between the twomachining operations.

In the drawings and specification, there has been set forth preferredembodiments of the invention, and although specific terms are employed,they are used in a generic and descriptive sense only and not forpurposes of limitation.

That which is claimed is:
 1. An endodontic instrument that is flexibleand resistant to torsional breakage and that is adapted for use inperforming root canal therapy on a tooth, comprising:a cylindricalelongate shank composed of an alloy comprising at least about 40%titanium and having a diameter not greater than about 0.07 inches, saidelongate shank further having a proximate end and an opposite pilot endso as to define a working length adjacent said pilot end; and at leastone ground flute extending helically around said shank working length,said one flute defining at least one cutting edge, and with the cuttingedge being formed by moving the shank past a grinding wheel only onceand which defines an as-ground condition of said cutting edge which issharp and substantially free of rolled deformed metal.
 2. The endodonticinstrument as defined in claim 1 wherein said shank is composed of analloy comprised of at least about 40% titanium and at least about 50%nickel.
 3. The endodontic instrument as defined in claim 1, wherein theshank is tapered.
 4. The endodontic instrument as defined in claim 1wherein said one flute is ground to form a surface that is linear whenviewed in transverse cross section.
 5. The endodontic instrument asdefined in claim 1 wherein said one flute is ground to form a surfacethat is arcuate when viewed in transverse cross section.
 6. Theendodontic instrument as defined in claim 5 wherein a helical land ispositioned between axially adjacent flute segments.
 7. The endodonticinstrument as defined in claim 1 further comprising a handle mounted atsaid proximate end of said shank.